This invention relates to a device and method of performing a pull test on miniature metallic bonds of electrical equipment.
A substrate for use in electrical apparatus, such as a cell phone, typically defines electrical pathways for connecting electrical components thereof. In miniature devices electrical connections to the substrate are made via soldered or welded connections, and for this purpose metallic balls, for example of solder, are formed on the component and re-flowed or welded when assembled to a mating substrate.
Typically a component may be in the range 5-50 mm and have solder balls thereon. Such components are often termed BGA's (ball grid arrays). These balls have the appearance of a low circular dome or squashed sphere, and have a diameter in the range 0.1-1.0 mm.
It is necessary to test the mechanical strength of the bond between the solder ball and the substrate in order to give confidence that the production bonding method is adequate, and that the bond strength is sufficient. One kind of test applies a tension load to the solder ball by gripping and pulling. In use a strong bond will result in ductile failure of the solder ball, with progressive deformation until the solder ball breaks away; part of the solder ball remains adhered to the substrate. A weak bond will typically exhibit brittle failure and tear away from the substrate leaving little residue adhered thereto.
The very small size of solder balls, and/or the low detected forces have resulted in the development of specialist test equipment. Such equipment may be semi-automated so that successive balls on a component are indexed one by one to a test position, for pull testing.
In particular, devices have been developed with jaws to grip a solder ball so as to exert a pulling (tension) load. The pulling load is applied generally perpendicular to the bonding plane. Very low forces are detected by the use of special low friction techniques.
These very low forces can be successfully and accurately measured, using a force transducer, when the gripping head of the test device is moving slowly. However if high pull testing speeds are required (greater than 15 mm/sec), the inertial mass of the moving part(s) may mask the force required to break the mechanical bond. One aim of the present invention is thus to provide a low mass arrangement capable of exerting high gripping forces at the gripping head, and also capable of high speed testing.
A problem can exist with existing test devices where displacement of the gripping head is via a displacement member (or beam) having the force transducer thereon. If movement of the gripping head from an unclamped to a clamped condition on the sample being tested introduces strain into the displacement member, that strain will be measured by the force transducers and affect the absolute values of tensile stress recorded during testing. For example a slight initial strain on clamping may skew tensile values notwithstanding that the strain gauge output is zeroed prior to commencement of the test. It is desirable to de-couple to the greatest extent, displacement of the gripping head during clamping and force measurement. This is achieved in the apparatus of the present invention as described in more detail below. This feature is advantageous both for low and high speed testing.
Yet another potential difficulty is to maximize the potential testing speed. Stiffness of the gripping head is an important factor because if the chosen testing speed is of the same order as the resonant frequency of the gripping head, the test results will be inaccurate. Therefore, the gripping head should be stiff. However since force measurement conventionally relies upon strain of a beam, i.e. bending, too much stiffness will reduce sensitivity. Accordingly, a compromise which allows a gripping head with high resonant frequency and high stiffness is desirable, resulting in a system which has a suitable bandwidth for the high speed testing, but can also be used for conventional low speed testing.
Typically ‘high speed’ means greater than 15 mm/s, and as high as 1000 mm/s. Conventional testing, in which a gripper pulls a deposit off a substrate, is typically in the range 0.1-15 mm/s, and is referred to in this specification, comparatively, as ‘low speed’ testing.